Since I didn't use the system for some time, there were many outdated packages.
At first I updated Mesa 18.0.1 and it's dependencies only and not any other packages. After the installation I rebooted and logged in to KDE. After a few seconds of doing nothing, the issue reappeared and the whole system froze. I had to hard reset the system.
Afterwards I retried after installing all outdated packages. This time the system froze immediately after the login to KDE: Black screen, no cursor, no tty switch (ALT + F-Keys).